The phase behavior at 298.2 K of a three-component system of 1,3,5-trimethylbenzene (mesitylene), water, and a nonionic surfactant of the nonyl phenyl polyoxyethylene type (Berol 227) was investigated. The microstructure of the isotropic oil-surfactant-rich region in this system was examined by means of conductivity and viscosity measurements. At relatively low surfactant contents, open or bicontinuous structures are formed, but as the anionic surfactant sodium 1,4-bis(2-ethylhexyl)sulfosuccinate is added, dosed aggregates are formed. Some compositions of this pseudo-three-component system were tested as cocktails for liquid scintillation counting. The counting efficiency seems to be independent of the microstructure in the isotropic oil-surfactant-rich phase, but as the one-phase border is crossed and birefringent phases are formed, the counting efficiency steeply decreases. When a two-phase system of isotropic solutions is formed by crossing the one-phase border, no abrupt change in the counting efficiency is recorded. At relatively low surfactant contents, the counting efficiency is approximately at the same level as for the commercially available scintillation cocktail Supermix.